In an audio reproduction system in which a speaker reproduces sound signals such as music and an announce sound, sound distortion or cracking sound may occur owing to the input signal exceeding a reproduction limit of the speaker, and the sound quality may be deteriorated. This will be described in detail below.
In the reproduction using a speaker, since a diaphragm of the speaker is allowed to vibrate within its maximum displacement amplitude, when a signal causing the diaphragm displacement to exceed the maximum displacement amplitude is inputted, the speaker diaphragm cannot vibrate properly, so that sound distortion and cracking sound occur. The displacement amplitude of a speaker diaphragm depends on the frequency of an input signal. This relation is shown in FIG. 8. FIG. 8 is a schematic graph showing the displacement amplitude of a speaker diaphragm when a signal is inputted to a speaker under the condition that only the frequency of the signal is changed with the voltage (V) kept constant. In reality, however, the speaker's characteristic around the minimum resonance frequency F0 may be somewhat different from that in FIG. 8, depending on the Q factor, etc. indicating the extent of the damping of the speaker. But, roughly speaking, the characteristic is the same. Further, it is noted that the present invention can be applied even to a speaker whose displacement amplitude characteristic is different from that shown in FIG. 8. However, for convenience of description, the characteristic shown in FIG. 8 will be used as an example.
As shown in FIG. 8, the displacement amplitude of the speaker diaphragm is substantially constant at frequencies below F0 and decreases with a slope of approximately −12 dB/oct at frequencies above F0. This indicates that the speaker diaphragm vibrates with much displacement amplitude when a frequency lower than around F0 is inputted to the speaker when compared with a case where a higher frequency is inputted to the speaker. Thus, when a signal including many low frequency components is inputted to the speaker and the voltage of the signal is increased, the maximum displacement amplitude of the diaphragm is exceeded at a voltage equal to or larger than a certain voltage. In other words, the reproduction limit of the speaker is more easily exceeded when the more the signal includes lower frequencies and the higher the voltage is increased. This situation is shown in FIG. 9.
In FIG. 9, the vertical axis represents the amplitude of a signal and the horizontal axis represents the frequency. Further, the gray pattern region represents a region where the cracking sound occurs because a displacement limit of the speaker diaphragm is exceeded, and the boundary is indicated by the bold line. Here, the characteristics shown in FIG. 9 are characteristics of audio signals with respect to amplitude values, and unlike the characteristic of the displacement amplitude of the speaker shown in FIG. 8, the displacement limit of the speaker diaphragm is represented by the slope of +12 dB/oct.
In addition, 901, 902, and 903 represent frequency characteristics of audio signals to be reproduced by the speaker, and in particular, cases are assumed where the signals include many low frequency components. Here, 901 is the characteristic at a low sound volume value, 902 is the characteristic at a medium sound volume value, and 903 is the frequency characteristic at a high sound volume value. When a reproduction is performed at the low sound volume value as in 901, the displacement limit of the speaker diaphragm is not exceeded even with the audio signal including many low frequency components, so that the cracking sound does not occur and the original quality sound can be enjoyed. Under an increased sound volume as in 902 and 903, however, the cracking sound occurs and the sound quality degrades because the displacement limit of the speaker diaphragm is exceeded.
As described above, when a signal exceeding the maximum displacement amplitude of the diaphragm is inputted, the speaker diaphragm cannot properly vibrate, and thus the cracking sound occurs.
Patent Document 1 discloses a conventional technique to reduce the cracking sound of a speaker. In Patent Document 1, an excessive input estimation unit, a control unit, and a frequency characteristic transformation unit are provided, and it is estimated that an audio signal for the reproduction leads to an excessive input, and then a variable filter is controlled in accordance with the estimation result to prevent the cracking sound of a speaker.